1. Field of the Invention
The present invention relates to a nitrogen oxide removal control apparatus and a method for reducing nitrogen oxide concentration in an exhaust gas by controlling an amount of ammonia which is injected to the exhaust gas of a gas turbine in a power generation plant.
2. Description of the Related Art
Recently, increasing demands for energy have caused a strong dependency on fossil fuel. As the energy supply amount owing to fossil fuel increases, CO.sub.2 exhaust amount has increased. Thus, the crisis of global warming has arisen and there is a worldwide move to put restrictions on the CO.sub.2 exhaust amount.
From the above view point, a combined power generation plant is expected to increase energy efficiency and allow reduction of CO.sub.2. This system is constructed by combination of a gas turbine and a steam turbine where steam is generated by using heat of exhaust gas of the gas turbine to drive the steam turbine.
As shown in FIG. 1, such a combined cycle power plant is provided with a gas turbine cycle 1, an heat recovery steam generator 3 for generating steam by using exhaust gas 2 of the gas turbine cycle 1 as a heat source, a steam turbine cycle 4 which uses this generated steam as a driving steam, and a chimney 5 for exhausting the heat-recovered exhausting gas. In the gas turbine cycle 1, fuel 8 supplied from a fuel system is combusted with air 6 compressed by an air compressor 7 in a combustor 9. Combustion gas generated drives a turbine 10. A generator 11 is connected to the turbine 10. After the exhaust gas 2 works, exhaust heat of the exhaust gas 2 is released to produce steam in the exhaust heat recovery steam generator 3 while the exhaust gas passes through an exhaust gas duct 12 to the chimney 5. The exhaust heat recovery steam generator 3 has a superheater 13, an evaporator 14, a nitrogen oxide removal device 15 and an economizer 16 along the upstream side to the downstream side of the exhaust gas duct 12. Steam generated in the superheater 13 is supplied to the steam turbine cycle 4 via a steam tube 17. In the steam turbine cycle 4, the steam coming out of a turbine 18 is condensed by a condenser 20. The condensate is introduced to the economizer 16 by a feed water tube 21, heated therein, evaporated in the evaporator 14, and the steam is further heated in the superheater 13. In the evaporator 14, while feed water is forcedly circulated or naturally circulated by temperature difference, heat-absorption and evaporation are effected. In FIG. 1, although the turbine 18 is connected to a generator 19, both the turbines 10 and 18 may be connected to the same generator to construct single shaft type combined cycle power plant.
In a plant with a gas turbine cycle 1 such as above combined cycle power plant, firing temperature is preferably higher to increase the plant efficiency and relatively reduce CO.sub.2. However, as firing temperature increases, nitrogen oxide (NO.sub.x) emitted from a gas turbine cycle 1 increases exponentially with the increasing temperature. Since this nitrogen oxide (NO.sub.x) is recognized as one contributing factor in air pollution, strict standards are applied to its emission.
Illustrative methods for reducing NO.sub.x concentration are a method where water or steam is injected to a combustor 9 to decrease firing temperature, a method where fuel and air is mixed in advance and then the mixture is introduced to the combustor 9 to prevent a partial higher part, and a method where a multistaged combustor capable of averaging combustion temperature is used.
However, it is difficult with only these methods to achieve the NO.sub.x emission standards. Thus, the nitrogen oxide removal device 15 is provided in a flow path of exhaust gas to reduce NO.sub.x emission. There is an ammonia injection/dry selective catalytic reduction decomposition method as one nitrogen oxide removal method applied to in this nitrogen oxide removal device. In the method, ammonia is injected to exhaust gas and the exhaust gas is passed through catalyst 22 placed on the downstream side of the injection point so that nitrogen oxide is reduced and decomposited to non-toxic nitrogen component and water steam. Generally, this method has good reaction efficiency at 300.degree. C. to 400.degree. C. based on the temperature properties of the catalyst. Accordingly, the nitrogen oxide removal device 15 using this method is placed between the evaporator 14 and the economizer 16.
In this nitrogen oxide removal device 15, NO.sub.x removal is controlled by adjusting an ammonia injection amount from an ammonia injection system 23. U.S. Pat. No. 4,473,536 and U.S. Pat. No. 4,473,537 disclose control system where a mole ratio of ammonia to NO.sub.x is obtained by proportional integral (PI) control based on a deviation of a set NO.sub.x value and a measured NO.sub.x value, and this is multiplied by a calculated predicted NO.sub.x value at an inlet of the nitrogen oxide removal device to obtain an ammonia injection amount. However, in this control system, since a predicted NO.sub.x value is multiplied, loop gain varies dependently on the predicted NO.sub.x value. If the predicted NO.sub.x value becomes smaller, there is a tendency that loop gain is also decreased to degrade response.
Further, the other following control system has already been known. In this system, when load of a gas turbine and the like do not change, an ammonia flow amount is controlled by feedback control loop which does proportional integral operation based on a deviation of a set NO.sub.x value and a measured NO.sub.x value. When disturbance such as a load change of a gas turbine, which effects NO.sub.x generation, is detected, an ammonia amount based on the detected disturbance amount is obtained by feedforward control loop. This ammonia amount, which use as a feedforward control signal, is added to a feedback control signal obtained by proportional integral operation of a deviation of a set NO.sub.x value and a measured NO.sub.x value, thereby controlling an ammonia flow amount. A delay time is about four minutes from a time when an ammonia flow amount adjustment valve is opened or closed to a time when this opening or closing influences a measured NO.sub.x value. On the contrary, it takes about a second or less that exhaust gas of a gas turbine passes from the gas turbine to a chimney. Thus, when there is disturbance such as a load change of the gas turbine, an ammonia flow amount cannot be controlled by the above-mentioned feedback control loop. For this reason, according to this control system, when disturbance such as a load change of the gas turbine is detected, a relay is activated, a contact of an output part of the above-mentioned feedforward control loop is closed, and a contact between a proportional operating unit and an integral operating unit of the feedback control loop is opened. As a result, input to an integral controller is stopped to prevent unnecessary history from remaining in the integral operating unit. However, a single shaft type combined cycle of a gas turbine and steam turbine has problems that it is difficult to exactly detect load change of a gas turbine and a load change detecting relay, which switches opening/closing of each control loop contact, does not always exactly operate.